The present invention relates to an LED lamp and, particularly, to an LED lamp for a traffic signal.
As light emitting diodes capable of emitting light of R, G and B primary colors and light emitting diodes capable of emitting white light with high luminance have been developed, LED lamps including a plurality of light emitting diodes arranged in an array have been put in use for various applications. The LED lamp has a far higher service life than that of an incandescent lamp, and also shows a high efficiency and a high resistance against vibration. Due to these advantages, the LED lamp has been used in advertising sign boards, traffic sign boards displaying route guide or traffic information, and light source for traffic signals and large screens.
With regard to the application of the LED lamp to traffic signals, in particular, while the incandescent lamp used as the light source of the conventional traffic signal requires large reflector mirrors and color filters, the LED lamp has such advantages as the capability to emit light of a single color that eliminates the need for a color filter and the capability to emit light with some degree of directivity that eliminates the need to install a large reflector mirror.
Moreover, a traffic signal constituted from LEDs that does not need reflector mirrors and color filters also has an advantage of being free from spurious lighting which is caused by extraneous light that has entered the traffic signal and been reflected off the reflector mirror placed behind an incandescent lamp, coming out of the traffic signal through a color filter.
The LED lamp used in such a traffic signal or a sign board is usually installed at an overhead location so as to be recognized by many people from a distance. As such, the LED lamp is required to emit light with symmetrical intensity distribution in the horizontal plane but asymmetrical intensity distribution in the vertical plane so that light intensity is higher in the front field and the lower field.
As it has been made possible to increase the luminous intensity of light emitting diodes recently, it is possible to decrease the number of light emitting diodes required in an LED lamp. However, a new problem has arisen in that it is difficult to achieve planar light emission of uniform intensity with an LED lamp consisting of a small number of light emitting diodes that have high luminous intensity.
The object of the present invention is to provide an LED lamp that is capable of achieving a desired luminous intensity distribution characteristic.
The second object of the present invention is to provide an LED lamp that is capable of achieving a desired luminous intensity distribution characteristic and producing uniform planar light emission.
In order to achieve the objects described above, the LED lamp of the present invention comprises a plurality of light emitting diodes and a cover lens so that light emitted by the plurality of light emitting diodes is output through the cover lens.
The cover lens has a plurality of prisms including a plurality of first prisms formed on the inner surface thereof. The direction and shape of the incident surfaces of the first prisms are set such that the first prisms output light in first luminous intensity distribution patterns that are substantially equal to each other.
The LED lamp of the present invention that has the constitution described above can emit light from the plurality of light emitting diodes in a luminous intensity distribution pattern that corresponds to the direction and shape of the incident surfaces of the first prisms.
The LED lamp of the present invention also allows the direction in which light that is output through the first prisms emerges in the vertical plane to be set by the direction of the incident surface, and the direction in which light that is output through the first prisms diffuses in the horizontal plane to be set by the configuration of the incident plane.
The LED lamp of the present invention preferably also includes a plurality of second prisms, and the direction and shape of the incident surfaces of the second prisms are set so that the second prisms have second luminous intensity distribution patterns that are substantially equal to each other.
Since the desired luminous intensity distribution pattern of the LED lamp can be achieved by means of the first and second prisms in the constitution described above, a better luminous intensity distribution pattern that is optimized can be achieved than in the case where the luminous intensity distribution pattern is formed with the first prisms only.
This constitution allows the direction in which light that is output through the second prisms emerges in the vertical plane to be set by the direction of the incident surface, and the direction in which light that is output through the second prisms diffuses in the horizontal plane to be set by the configuration of the incident surface.
In the LED lamp of the present invention, it is preferable that the first prisms and the second prisms form a pair, and the pair of prisms are arranged on the inner surface of the cover lens according to a certain rule, thereby achieving a uniform planar light emission on the external surface of the cover lens.
In the LED lamp of the present invention, the second prisms can be formed so as to direct the light below the first prisms, which makes it possible to provide an LED lamp that is required to emit light with higher intensity in the lower portion.
Further the plurality of light emitting diodes are preferably disposed in an arrangement pattern determined according to the luminous intensity distribution pattern required of the LED lamp. Since the desired luminous intensity distribution characteristic of the LED lamp can be achieved based on the arrangement pattern of the plurality of light emitting diodes, the first luminous intensity distribution pattern and the second luminous intensity distribution pattern in the constitution described above having a better luminous intensity distribution pattern that is optimized to provide the luminous intensity distribution characteristic described above.
Also in the LED lamp of the present invention, the arrangement pattern is preferably set such that the number of light emitting diodes located below the horizontal plane that includes the central axis of the cover lens is set to 40% or less of the total number of the light emitting diodes. Thus, the LED lamp that is required to emit light with higher intensity in the lower portion can be made easily.
Also in the LED lamp of the present invention, the angle between the line that connects the light emitting diode located at the outermost position of the arrangement pattern and the periphery of the cover lens on the inner surface thereof and the central axis of the cover lens is preferably in a range from 30xc2x0 to 60xc2x0. With this constitution, uniform planar light emission can be achieved on the external surface of the cover lens.
Moreover, in the LED lamp of the present invention, it is preferable that the cover lens has an external surface that is a spherical surface having a radius of curvature of 500 mm or less.
With this constitution, when the first prisms or both the first prisms and the second prisms are used, light emitted by the light source can be output more efficiently. Also, it is possible to mitigate the reflection of extraneous light on the cover lens surface, thereby preventing the contrast from decreasing when lighted.
As described above, the present invention can provide an LED lamp that is capable of achieving a desired luminous intensity distribution characteristic and producing uniform planar light emission.